U.S. patent number 6,258,480 [Application Number 09/284,304] was granted by the patent office on 2001-07-10 for battery and method of manufacturing therefor.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Mamoru Iida, Akira Iwase, Susumu Kitaoka, Isao Matsumoto, Yoshio Moriwaki.
United States Patent |
6,258,480 |
Moriwaki , et al. |
July 10, 2001 |
Battery and method of manufacturing therefor
Abstract
A battery accommodates elements for electromotive-force within a
metal case. This metal case is a metal case having a bottom wherein
the bottom thickness/side thickness ratio has a value of 1.2-4.0
and has a cylindrical, prismatic or similar shape. The metal case
is constructed of a metal material whose chief constituent is
aluminum. Furthermore, it is desirable that a multiplicity of
shallow grooves perpendicular to a bottom face are formed in at
least a battery inside face of the metal case and moreover that a
nickel layer is provided on the battery inside face. The metal case
is made by DI processing involving drawing and ironing, to have a
value of bottom thickness/side thickness ratio which was hitherto
unavailable,i.e., 1.2-4.0 can be obtained.
Inventors: |
Moriwaki; Yoshio (Hirakata,
JP), Iwase; Akira (Hirakata, JP), Kitaoka;
Susumu (Hirakata, JP), Iida; Mamoru (Kadoma,
JP), Matsumoto; Isao (Osaka, JP) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JP)
|
Family
ID: |
17099104 |
Appl.
No.: |
09/284,304 |
Filed: |
April 9, 1999 |
PCT
Filed: |
September 02, 1998 |
PCT No.: |
PCT/JP98/03942 |
371
Date: |
April 09, 1999 |
102(e)
Date: |
April 09, 1999 |
PCT
Pub. No.: |
WO99/13520 |
PCT
Pub. Date: |
March 18, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Sep 8, 1997 [JP] |
|
|
9-243120 |
|
Current U.S.
Class: |
429/176;
29/623.1; 29/730; 429/185; 429/163 |
Current CPC
Class: |
H01M
50/116 (20210101); H01M 50/56 (20210101); H01M
50/10 (20210101); H01M 50/1243 (20210101); H01M
50/103 (20210101); Y10T 29/49108 (20150115); H01M
10/0525 (20130101); Y10T 29/53135 (20150115); H01M
6/10 (20130101); Y02E 60/10 (20130101) |
Current International
Class: |
H01M
2/02 (20060101); H01M 10/40 (20060101); H01M
10/36 (20060101); H01M 6/04 (20060101); H01M
6/10 (20060101); H01M 002/04 () |
Field of
Search: |
;429/176,164,163,167,168,185,177,174 ;72/349,46 ;29/623.1,730 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
60-180058 |
|
Sep 1985 |
|
JP |
|
63-37066 |
|
Mar 1988 |
|
JP |
|
2-150660 |
|
Dec 1990 |
|
JP |
|
7-99686 |
|
Oct 1995 |
|
JP |
|
8-162074 |
|
Jun 1996 |
|
JP |
|
8-255598 |
|
Oct 1996 |
|
JP |
|
8-329908 |
|
Dec 1996 |
|
JP |
|
Primary Examiner: Brouillette; Gabrielle
Assistant Examiner: Alejandro; Raymond
Attorney, Agent or Firm: Jordan and Hamburg LLP
Claims
What is claimed is:
1. A battery comprising:
elements for electromotive-force;
a metal case which has a bottom and a side wall extending to a top
edge defining a sealing aperture, said elements for
electromotive-force being disposed in said metal case;
said bottom having a bottom thickness and said side wall having a
side thickness wherein a ratio of said bottom thickness to said
side thickness is in the range of 1.2-4.0; and
said metal case being formed of one of a metal material which is
comprised substantially of aluminum and an alloy material which is
comprised substantially of aluminum, wherein a Vickers hardness
value of said side wall formation of said metal case has a value at
least 1.2 times a Vickers hardness value of the metal material or
the alloy material prior to formation into said case.
2. The battery of claim 1, wherein said side walls have an inside
face defining a multiplicity of shallow grooves extending
perpendicular to said bottom.
3. The battery of claim 2, wherein said shallow grooves have a
depth which is in the range of 0.5-10.0 .mu.m.
4. The battery of claim 1, further comprising the metal case having
a nickel layer of a thickness less than 30 .mu.m provided on at
least one of an inside surface and an outside surface of said metal
case.
5. The battery of claim 1 , wherein said side wall has a side
thickness at said sealing aperture which is at least 10-30% thicker
than another side thickness of said side wall at another
location.
6. The battery of claim 1, wherein said metal case has a
substantially prismatic shape and interior corners of a radius of
curvature less than 0.5 mm.
7. The battery of claim 6 wherein said side wall includes side wall
sections adjoining one another at ones of said interior corners and
said side wall adjoins said bottom at other ones of said interior
corners.
8. A method of manufacturing a battery comprising the steps of:
providing a sheet of metallic material which is comprised
substantially of aluminum and an alloy material which is comprised
substantially of aluminum;
drawing forming said sheet into a metal case having a tubular shape
with a bottom having a bottom thickness and a side wall having a
side wall thickness and a top edge defining a sealing aperture;
ironing said side wall to have an ironing ratio in the range 10-80%
and a ratio of said bottom thickness to said side thickness is in
the range of 1.2-4.0;
forming a multiplicity of shallow grooves on an inside face of said
side wall perpendicular to said bottom;
disposing elements for producing electromotive force in said metal
case; and
sealing said sealing aperture.
9. The method of manufacturing a battery of claim 8, further
comprising the step of providing a nickel layer on at least one of
an exterior surface and an interior surface of said metal can.
10. The method of manufacturing a battery of claim 8 or 9, wherein
said ironing is performed continuously such that the ironing ratio
is in the range of 30-80%.
11. The method of claim 8 wherein said multiplicity of shallow
grooves are formed during said drawing.
12. The method of claim 8 wherein said multiplicity of shallow
grooves are formed during said drawing by introducing hard
particles.
13. The method of claim 8 wherein said hard particles are
alumina.
14. The method of claim 8 wherein said providing said nickel layer
includes providing said sheet with said nickel layer disposed
thereon.
15. The method of claim 8 wherein said multiplicity of shallow
grooves depth which is in the range of 0.5-10.0 .mu.m.
16. The method of claim 8 wherein the nickel layer has a thickness
less than 30 .mu.m.
17. The method of claim 8 wherein said ironing produces said side
wall has a side thickness at said sealing aperture which is at
least 10-30% thicker than another side thickness of said side wall
at another location.
18. The method 8 wherein said metal case is formed by said drawing
and said ironing to have a substantially prismatic shape and
interior corner of a radius of curvate less than 0.5 mm.
19. The method of claim 18 wherein said side wall includes side
wall section adjoining one another at ones of said interior corners
and said side wall adjoins said bottom at other ones of said
interior corners.
20. The method of claim 8 wherein said bottom thickness is 0.5
mm.
21. The method of claim 20 wherein said ironing ratio is 30%.
22. The method of claim 20 wherein said ratio of said bottom
thickness to said side thickness is 1.43.
23. The method of claim 8 wherein said bottom thickness is 0.5
mm.
24. The method of claim 8 wherein said drawing and said ironing
provides a Vicker hardness value of said side wall after formation
of said metal case that is at least 1.2 times a Vicker hardness
value of the metal material prior to formation into said metal
case.
25. The method of claim 24 wherein said drawing and said ironing
provides a Vickers hardness value of said side wall of 71.
26. The method of claim 8 wherein said drawing and said ironing
provides a Vickers hardness value of said side wall after formation
of said metal case that is 2.37 times a Vicker hardness value of
the metal material prior to formation into said metal case.
27. The method of claim 24 wherein said drawing and said ironing
provides a Vickers hardness value of said side wall of 58.
28. The method of claim 8 wherein said drawing and said ironing
provides a Vickers hardness value of said side wall after formation
of said metal case that is 1.93 times a Vicker hardness value of
the metal material prior to formation into said metal case.
Description
TECHNICAL FIELD
The present invention relates to a battery such as a primary
battery or secondary battery and in particular relates to
improvements in the outer metal jacket (metal case) of a
cylindrical or prismatic battery.
BACKGROUND ART
In recent years, as portable devices have become increasingly
widespread, demand for miniature primary batteries and secondary
batteries has increased. The chief types of primary batteries are
manganese dry batteries or alkali manganese dry batteries or
lithium batteries, and large numbers of these are used depending on
the respective application. Also, as secondary batteries,
considerable use has hitherto been made of nickel-cadmium
accumulators, which are alkali accumulators in which an aqueous
solution of alkali is employed as the electrolyte, and
nickel-hydrogen rechargeable batteries, in which a
hydrogen-absorption alloy is employed as the negative electrode.
Recently however, lithium ion secondary batteries, which are
characterized by light weight and high energy density, and employ
an organic electrolyte, have suddenly appeared on the market.
Chiefly in the case of miniature secondary batteries for portable
equipment, in addition to the cylindrical type and coin type, which
were the typical conventional battery shapes, in recent years use
of batteries of prismatic shape has increased, and, most recently,
paper-form thin batteries have also appeared.
An important recent trend in the demands made on performance of
such batteries is increasing demand for higher energy density of
the battery. In general terms, there are two methods of indicating
the energy density of a battery. One of these is volumetric energy
density (Wh/l); this is used as an index of battery
miniaturization. Another is weight energy density (Wh/kg); this is
used as an index of battery weight reduction.
Batteries of high volumetric energy density and high weight energy
density, these respectively being indices of miniaturization and
weight reduction, are highly prized by the market and there is
fierce competition to increase the energy density of all types of
battery.
What determines the level of energy density of a battery is
principally the battery active materials of the positive electrode
and/or negative electrode constituting the elements for
electromotive-force, but apart from these the electrolyte and
separators are also important. Very vigorous efforts are currently
being made to improve these elements for increasing the energy
density of the battery.
Meanwhile, miniaturization and weight reduction of the battery
casing, i.e., the case of the battery that accommodates these
elements for electromotive-force, which previously tended to be
overlooked, has been in recent years re-evaluated as an important
question and positive efforts are being made to achieve
improvements in this respect. If the case of the battery can be
made thinner, more battery active material can be accommodated in a
portion of the same shape as conventionally but of reduced
thickness, enabling the volumetric energy density of the battery as
a whole to be raised. Also, if the battery case can be made of
lighter material of lower specific gravity, the weight of the
battery as a whole can be reduced by lowering its weight for the
same shape as conventionally, and the weight energy density of the
battery as a whole can thereby be raised.
Adoption of the DI (Drawing and Ironing) technique for the battery
case is noteworthy as a previous technique for improving volumetric
energy density. Conventionally, drawing processing was chiefly
employed for manufacturing battery cases using iron-based metal
material, but recently the DI technique, using both drawing and
ironing, has attracted attention. Known methods for manufacturing a
battery case are the technique (hereinbelow called "drawing-only
technique") in which a battery case of prescribed shape is
manufactured by repeating a plurality of deep-drawing steps using a
press, and the so-called "DI technique", which is a technique in
which a cylindrical battery case of prescribed shape is
manufactured from a cup-shaped intermediate product obtained by
manufacturing a cup-shaped intermediate product by a deep-drawing
step using a press, followed by an ironing step using an ironing
machine; this technique is known from Japanese Patent Publication
No. 7-99686 etc.
Compared with the "drawing-only technique", the "DI technique" has
the advantages of increased productivity due to diminution in the
number of process steps, weight reduction and increased capacity
due to reduction in thickness of the circumferential walls of the
case, and reduction in stress corrosion etc., and for these reasons
its rate of utilization is increasing. Also, conventionally, in the
above method of manufacture, nickel-plated steel sheet, which is of
comparatively high hardness, was employed as the battery case blank
material in order to ensure sufficient pressure-resisting strength
of the battery case and sufficient strength of the sealing portion.
This DI technique enables the thickness of the case walls to be
reduced and is said to make possible an improvement in volumetric
energy density of the battery of about 5%.
Also, a well known example in which the battery case is changed to
a lightweight material of lower specific gravity is provided by the
case of prismatic lithium batteries, in which aluminum alloy sheet
(specific gravity: about 2.8 g/cc) is employed instead of the
conventional rolled steel sheet (specific gravity: about 7.9 g/cc).
Efforts have been made towards weight reduction of batteries for
use in portable telephones and, as a result, in this case also,
examples are known in which an improvement of about 10% in weight
energy density of the battery as a whole has been achieved by
weight reduction of the case by changing the blank material to
aluminum alloy. An example of a secondary battery using such an
aluminum case is disclosed in Japanese Patent Laid-Open No.
8-329908. Impact processing or drawing processing have frequently
been used as methods of manufacturing battery cases using aluminum
or aluminum alloy.
Although there is some variation depending on battery size, if
cold-rolled steel sheet is employed, the weight ratio represented
by the case to that of the overall battery weight in batteries that
have been practically employed up to the present is about
10.about.20 wt. % in the case of a cylindrical nickel/hydrogen
rechargeable battery or lithium ion secondary battery; in the case
of a prismatic nickel/hydrogen rechargeable battery or lithium ion
secondary battery, this is about 30.about.40 wt. % i.e. twice the
value for the cylindrical type. Recently, by employing aluminum or
aluminum alloy for the case of prismatic lithium ion secondary
batteries, this value has been reduced to 20.about.30 wt. %.
While these trends to miniaturization and weight reduction of the
battery case are effective in improving battery energy density, on
the other hand, in batteries, chemical reactions involving changes
in the substances in the charging or discharging reaction are
employed, and reliability of quality and safety therefore
constitute properties which are just as important in use as energy
density and cannot be neglected. In the case of primary batteries
that are employed exclusively for discharge, guaranteeing capacity
and/or prevention of leakage over a long period of storage, and
reliability of qualities such as stable discharge performance are
indispensable. In the case of secondary batteries that perform
repeated charging and discharging, in addition to the properties
demanded for primary batteries, performance such as cycle life and
safety are even more important.
Conventionally, it was extremely difficult to maintain both high
energy density and quality reliability together with safety in
respect of such battery cases. Specifically, if it was attempted to
obtain high energy density, deformation of the battery case or
cracking under abnormal conditions frequently gave rise to problems
such as leakage of electrolyte. On the other hand, if the case was
made strong, this often resulted in high energy density being
sacrificed; an effective method of improving the trade-off
relationship between these two had not been discovered.
In the techniques for manufacturing a case as indicated above, a
method based on the DI technique using drawing and ironing is
excellent in that it enables relative satisfaction of both improved
battery energy density i.e. thin walls and light weight and battery
quality reliability together with safety. However, in this
connection, further improvement in performance and quality
reliability together with safety has been demanded.
Demands for such battery miniaturization and weight reduction in
the market for primary batteries and secondary batteries is strong
and more convenience is also sought. On the other hand, quality
reliability and safety of such batteries are indispensable;
previously, both of these two, namely, improved battery energy
density making possible battery miniaturization and weight
reduction, and quality reliability and safety, were insufficiently
satisfied.
Also, regarding the technique of manufacturing the case of
aluminum-based metal material, with the conventional method,
reduction in thickness of the case walls was insufficient and, as a
result, miniaturization and weight reduction of the battery was
insufficient.
The present invention was made in the light of the above problems.
Its object is to provide a battery and method of manufacturing it
whereby miniaturization and weight reduction of the case of
cylindrical shape or prismatic shape or shape similar thereto
employed in primary batteries or secondary batteries can be
achieved and the energy density of the battery can be raised, and
also in which battery quality reliability and safety can be
satisfied.
DISCLOSURE OF THE INVENTION
The present invention provides a battery accommodating elements for
electromotive-force within a metal case; this metal case being a
metal case having a bottom wherein the bottom thickness/side
thickness ratio has a value of 1.2.about.4.0 and having a
cylindrical, prismatic or shape similar thereto; this metal case
being constructed of a metal material whose chief constituent i. e.
substantially composed of, is aluminum, or an alloy material whose
chief constituent is aluminum. Also, in the above, it relates to a
battery wherein a multiplicity of shallow grooves perpendicular to
the bottom face are formed in at least the battery inside face of
the metal case or wherein the depth of the grooves formed in the
battery inside face is 0.5.about.10.0 .mu.m.
Also a battery as aforesaid may include in a battery wherein the
metal case is constituted of a metallic material whose chief
constituent is aluminum or an alloy material whose chief
constituent is aluminum, and a nickel layer of thickness less than
30 .mu.m is provided on at least either the battery inside face or
outer face.
The present invention further includes in a method of manufacturing
a battery in which a sheet of metallic material, whose chief
constituent is aluminum, or a sheet of alloy material, whose chief
constituent is aluminum, is subjected to continuous ironing
processing (DI processing) form into a side part of a case formed
in a tubular shape having a bottom, such that the ironing ratio
(where the ironing ratio (%) is defined as follows: ironing ratio
(%)=(original thickness-thickness after ironing).times.100/original
thickness) is in the range, 10.about.80%, and wherein a metal case
having a bottom and being of a cylindrical shape, prismatic shape
or shape similar thereto, which is formed with a multiplicity of
shallow grooves perpendicular to the bottom face in the battery
inside face and which has a value of bottom thickness/side
thickness of 1.2.about.4.0 is thereby manufactured. In this case,
preferably for the metallic material sheet whose chief constituent
is aluminum or the alloy material sheet whose chief constituent is
aluminum, a sheet provided with a nickel layer on at least either
the battery inside face or outside face is employed and ironing is
performed continuously such that the ironing ratio is in the range
30.about.80%.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing a cross-section of a metal case of
cylindrical shape having a bottom employed in an embodiment of the
present invention;
FIG. 2 is a process diagram showing manufacturing steps of above
metal case;
FIG. 3 is a view comparing the high-rate discharge characteristics
of a cell A used in an embodiment of the present invention and a
cell B according to a comparative example;
FIG. 4 shows a metal case of prismatic shape having a bottom used
in a further embodiment of the present invention, (a) being a
longitudinally-sectioned front view, (b) being a
longitudinally-sectioned side view, (c) being a plan view, (d)
being a view to a larger scale of the portion indicated by P in
(c), and (e) being a view to a larger scale of the portions
indicated by Q.sub.1 and Q.sub.2 respectively in (a) and (b).
DETAILED DESCRIPTION
A battery according to the present invention is a battery
accommodating elements for electromotive-force within a metal case.
The metal case is a metal case having a bottom wherein the bottom
thickness/side thickness has a value of 1.2.about.4.0 and having a
cylindrical, prismatic or shape similar thereto. The metal case is
constructed of a metal material whose chief constituent is
aluminum, or an alloy material whose chief constituent is aluminum.
No conventional batteries using a metal case constituted of a
metallic material whose chief constituent is aluminum having a
cylindrical or similar form and whose bottom thickness/side
thickness has a value of 1.2.about.4.0 are known. Some examples of
batteries made using a metal case having a prismatic shape or shape
similar thereto are known, but, in all cases, the bottom
thickness/side thickness is less than 1.2. Batteries using a metal
case having a bottom thickness/side thickness with a value of
1.2.about.4.0 are not known. The present invention is characterized
in particular by DI processing using drawing and ironing of a metal
case. By this means, values of the bottom thickness/side thickness
which were not available conventionally are achieved. According to
the present invention, both high battery energy density, i.e., thin
walls and light weight, together with reliability of battery
quality and safety can be satisfied.
Also, in a battery according to the present invention, a
multiplicity of shallow grooves perpendicular to the bottom face
are formed in at least the battery inside face of the metal case,
in other words, a multiplicity of shallow grooves parallel to the
axial direction of the metal case are formed in the battery inside
face of the side walls of the metal case. In this case, the depth
of the grooves is preferably in particular about 0.5.about.10.0
.mu.m. The surface condition of the battery inside face of
conventional metal cases is comparatively flat, but, by forming the
battery inside face of a metal case according to the present
invention with a multiplicity of shallow grooves perpendicular to
the bottom face, the benefit is obtained that the electrical
contact resistance of the electrode plates, constituting the
elements for electromotive-force, and the metal case is enormously
reduced.
Also in these batteries, the metal case is constituted of a
metallic material whose chief constituent is aluminum or an alloy
material whose chief constituent is aluminum, and a nickel layer of
a thickness less than 30 .mu.m on at least either the battery
inside face or outer face. By providing a nickel layer of under 30
.mu.m thickness on the battery inside face, direct contact of the
aluminum of the blank material with the electrolyte is eliminated,
and, as a result, the benefit is obtained of increasing the
corrosion resistance of the metal case. Also, by providing a nickel
layer of under 30 .mu.m on the outside face of the battery, the
lead connection strength can be raised when a battery pack is
constituted by connecting a plurality of cells.
Also, the HV value, indicating the Vickers hardness, of the side
walls of the metal case after formation of the metal case of the
aforementioned batteries should have a value of at least 1.2 times
the HV value of the metal material, whose chief constituent is
aluminum, or the alloy material, whose chief constituent is
aluminum, of the blank material used for the metal case; thus the
processing hardness value of the metal case is restricted.
Further, in the aforesaid batteries, regarding the thickness of the
side walls of the metal case, the side thickness in the vicinity of
the battery sealing aperture may be at least 10.about.30% thicker
than the side thickness in other areas. This is because when the
battery is used, the chief weakness in regard to pressure
withstanding strength, when the pressure within the battery rises,
is in the vicinity of the battery sealing aperture. It is therefore
possible to maintain the sealing strength by making the side
thickness in the vicinity of the battery sealing aperture, which is
usually inferior in its ability to withstand pressure, at least
10.about.30% thicker than the side thickness in other portions.
Further, in a battery as aforesaid, where the metal case has a
prismatic shape or shape similar thereto, the corner parts of the
battery inside face in the longitudinal cross-sectional plane and
transverse cross-sectional plane of this metal case may have a
curved shape of radius of curvature under 0.5 mm. By making the
radius of curvature of the corner parts of the battery inside face
less than 0.5 mm, the ability of the battery to withstand internal
pressure is raised and the elements for electromotive-force, such
as the positive electrodes, negative electrodes and separator, are
accommodated within the battery in less wasteful manner.
A method of manufacturing a battery according to the present
invention includes a method of manufacturing a battery in which a
sheet of metallic material, whose chief constituent is aluminum, or
a sheet of alloy material whose chief constituent is aluminum, is
subjected to drawing forming into a tubular shape having a bottom,
and continuous ironing is performed such that the ironing ratio of
the side of the case formed in the aforesaid tubular shape having a
bottom is in the range 10.about.80%, thereby manufacturing a metal
case having a bottom and which is of cylindrical, prismatic or
similar shape having a bottom thickness/side thickness of a value
of 1.2.about.4.0, formed with a multiplicity of shallow grooves
perpendicular to its bottom face on the battery inside face, this
being used to produce the battery. In this case, it is beneficial
if the metallic material sheet, whose chief constituent is
aluminum, or alloy material sheet whose chief constituent is
aluminum, is constituted by providing a nickel layer on at least
either the battery inside face or outside face. Preferably, also,
ironing is performed continuously such that the ironing ratio is in
particular in the range 30.about.80%.
The method of manufacture of a battery according to the present
invention has the benefit that a metal case having a bottom and
wherein the bottom thickness/side thickness has a value of
1.2.about.4.0 can be manufactured with a high ironing ratio of
metallic material sheet whose chief constituent is aluminum or
alloy material sheet whose chief constituent is aluminum.
Next, specific examples of the present invention is described.
As a first embodiment of the present invention, a lithium ion
secondary battery of cylindrical shape is described wherein the
metal case material is an alloy material, whose chief constituent
is aluminum, formed with a multiplicity of shallow grooves
perpendicular to its bottom face in at least battery inside face of
the metal case.
First of all, a metal case used in this battery is described with
reference to FIG. 1 and FIG. 2. As the alloy material whose chief
constituent is aluminum, from Al--Mn based alloys (3000 type) which
are wrought products of non-heat treated alloys, 3003 alloy was
selected. A sheet 2 of 3003 alloy of thickness 0.5 mm was first of
all punched out to circular shape and then subjected to drawing
using a press to manufacture a metal case cup 3 having a bottom of
external diameter 21.5 mm and height 15.5 mm. In the condition of
this cup, little change in either bottom thickness or side
thickness was found in comparison with the blank material.
Next, this metal case cup 3 having a bottom was introduced into a
DI metal mold, where a DI metal case having a bottom of external
diameter 13.8 mm, and a height 54.0 mm was manufactured by
continuous ironing. Since, in this condition, an upper side part
(lug) 5 of the metal case is not level, but has a somewhat
distorted shape due to the processing, a DI metal case having a
bottom, i.e., metal case 1 of external diameter 13.8 mm, height
49.0 mm was formed by cropping upper side part 5. FIG. 1 shows a
cross-sectional view of this metal case 1 having a bottom.
The thickness of the bottom wall 1a of this metal case 1 shown in
FIG. 1, i.e. a bottom thickness (TA) is 0.5 mm, and the thickness
of side wall 1b, i.e., a side thickness (TB), is 0.35 mm,
representing an ironing ratio of 30%. Also, bottom thickness
(TA)/side thickness (TB)=1.43. The side thickness (TB) indicated
here is the side thickness at intermediate height of metal case 1
and indicates the mean value of the side thickness. Furthermore, a
side thickness (this is called the sealing aperture vicinity side
thickness, TC) at a position 5 mm below the upper aperture,
constituting the sealing aperture vicinity portion 1c in the metal
case is indicated. Metal case 1 was manufactured such that the
sealing aperture vicinity side thickness (TC) was about 11%
thicker, at 0.39 mm, than the side thickness (TB) of the
intermediate portion, with the object of raising the sealing
aperture strength.
The HV value, indicating the Vickers hardness of the 3003 alloy
sheet before processing of the metal case was 30 and the HV value
of side wall 1b after forming the metal case was 71, i.e. the HV
value was increased by a factor of 2.37 by the DI processing.
According to the present invention, a multiplicity of shallow
grooves perpendicular to a bottom face are formed in a battery
inside face in the process of manufacturing a DI case by continuous
ironing. This multiplicity of shallow grooves perpendicular to the
bottom face on the battery inside face are scratch marks of the
metal mold in the DI case manufacturing process. Such scratch marks
can be formed by introducing comparatively hard particles such as
of alumina during DI processing. A multiplicity of shallow grooves
perpendicular to the bottom face can therefore easily be formed by
DI processing by forcibly dispersing alumina powder on the surface
of the inside face of the metal case cup having a bottom.
By observing the surface of the battery inside face of the metal
case produced by DI processing using a scanning electron
microscope, it was confirmed that a multiplicity of well-formed
grooves perpendicular to the bottom face were produced. In this
case, the depth of these grooves was, in particular, about
0.5.about.3 .mu.m. In this way, manufacture of a metal case
employed in a battery according to the present invention was
completed.
Next, a cylindrical lithium ion secondary battery was manufactured
using a metal case manufactured as described above. First of all, a
positive electrode and separator and negative electrode
constituting the elements for electromotive-force were prepared.
For the positive electrode, LiCoO.sub.2 and a conducting agent
including acetylene black, and a fluorinated resin binder etc.,
mixed into the form of a paste, was applied to an aluminum foil
substrate which was then dried, pressurized and cut to form an
electrode of prescribed dimensions. In order to effect direct
contact of this positive electrode with the metal case of the
battery, the positive electrode was provided with a portion
consisting solely of aluminum foil substrate. For the separator,
polyethylene micro-pore film of thickness 0.027 mm was employed.
For the negative electrode, a binder, styrene butadiene rubber
(SBR) and a thickener, carboxymethyl cellulose (CMC) etc. were
added to spherical graphite to form a paste, which was then applied
to a copper foil substrate which was then dried, pressurized and
cut to form an electrode of prescribed dimensions.
Next, the positive electrode and negative electrode were wound in
spiral fashion with interposition of a separator and accommodated
in the metal case referred to above. In this case, the outermost
peripheral portion, when thus wound in spiral fashion, is the
portion of the positive electrode consisting solely of aluminum
foil substrate, so the positive electrode terminal of the metal
case and the positive electrode plate are directly electrically
connected. Also, connection between the negative electrode terminal
constituted by a cap part of the sealed battery and the negative
electrode plate was effected by a nickel lead.
As the electrolyte, an electrolyte was employed obtained by
blending ethylene carbonate (EC)-diethyl carbonate (DEC) in a mol
ratio of 1:3 and dissolving lithium hexafluoro phosphate
(LiPF.sub.6) in a ratio of 1 mol/l. A sealed battery was obtained
by pouring this electrolyte into a battery, and sealing the metal
case and sealing aperture cap by ordinary laser aperture sealing.
This battery was of cylindrical type (size AA) of diameter 14 mm,
height 50 mm. The battery capacity was 600 mAh. This battery will
be referred to as cell A of this embodiment.
In order to compare the performance of cell A of this embodiment,
manufacture and evaluation of a cell B as a comparative example
were conducted. The differences between cell A of this embodiment
and cell B lay in the construction of the metal case.
Specifically, although cell B was the same as cell A in that 3003
alloy sheet of thickness 0.5 mm was used, the manufacture of its
case employed the drawing-only method; the bottom thickness of the
metal case obtained by this case-drawing was 0.5 mm, but the side
thickness was 0.43 mm, giving a value of the bottom thickness/side
thickness=1.16. Also, the battery inside face of the metal case of
cell B was comparatively flat, without formation of a multiplicity
of shallow grooves perpendicular to the bottom face.
The following could be said on comparing the performance of these
two cells A and B. Firstly, the side thickness of the metal case is
0.08 mm thicker in the case of cell B than in the case of cell A;
as a result, the effective volume for accommodating the elements
for electromotive-force of the battery was reduced by about 2.5% in
comparison with cell A; thus, the battery capacity of cell B was
585 mAh, representing a reduction in volumetric energy density of
about 2.5%.
Secondly, a difference in high-rate discharge performance was
found. FIG. 3 shows a comparison of the characteristics of
high-rate (1 CmA) discharge at 20.degree. C. As shown in FIG. 3, at
intermediate discharge voltage, the discharge voltage of cell B was
lower than that of cell A by about 30.about.50 mV at 1 CmA, and, as
a result, considerable problems were displayed under high-rate
discharge conditions occurring in actual battery use. In recent
years, in such lithium ion secondary batteries, high-rate discharge
characteristic is seen as important in practical use and a large
drop of voltage under fixed W discharge is a fairly serious
problem. In this regard, it is confirmed that cell A of this
embodiment shows a benefit of suppression of the drop of discharge
voltage under high-rate discharge conditions, thanks to the
formation of a multiplicity of shallow grooves perpendicular to the
bottom face in the battery inside face of the metal case.
It was confirmed that cell A of this embodiment had better
performance than cell B of the comparative example in respect of
energy density and high-rate discharge, as mentioned above. In
other evaluations, a marked difference was not found between the
two batteries.
Due to the above construction, according to the present invention,
compared with the case where iron-based steel sheet etc., which was
conventionally frequently employed as the metal case having a
bottom was used, the weight of the metal case itself can be made
lighter and the weight energy density of the battery can be greatly
raised. Also, due to the raised value of bottom thickness/side
thickness, iLt was found that processing hardening of the blank
material could be promoted and higher strength achieved in spite of
reduced thickness. In this way, a battery according to the
invention combines both the looked-for high battery energy density
and high reliability.
Next, as a second embodiment of the present invention, a prismatic
lithium ion secondary battery is described wherein the metal case
material is an alloy material whose chief constituent is aluminum,
a multiplicity of shallow grooves perpendicular to the bottom face
are formed in at least the battery inside face of the metal case,
and a nickel layer is provided on the battery inside face.
For the metal case used in the battery, 3003 alloy was selected
from Al--Mn based alloys (3000 type) constituting a wrought product
of non-heat treated alloy, as the alloy material whose chief
constituent is aluminum. A metal case cup having a bottom was
manufactured by drawing processing, by pressing a sheet of 3003
alloy of thickness 0.6 mm, both of whose faces had been
nickel-plated with a thickness of 5 .mu.m. In this cup condition,
little difference was found in the bottom thickness and side
thickness in comparison with the blank material.
Next, this metal case cup having a bottom was introduced into a DI
metal mold, and a DI metal case was manufactured having external
diameter dimensions: width 22 mm, height 52 mm, thickness 8 mm, by
continuous ironing. Since in this condition the top of the side
part (lug) of the metal case was not flat, but somewhat distorted
due to the processing, the upper part of the side was cropped to
obtain a metal case having a bottom of height 48 mm. As shown in
FIG. 4, the bottom thickness (TA) of this metal case 7 was 0.6 mm,
and the side thickness (TB) was 0.45 mm, representing an ironing
ratio of 25%. Also, the value of bottom thickness/side
thickness=1.33. The side thickness (TB) indicated herein is the
side thickness an intermediate height of the metal case 7 and
indicates the mean value of the side thickness.
In addition, the side thickness (this will be called the sealing
aperture vicinity side thickness, TC) at a position 5 mm below the
aperture at the top, constituting the sealing aperture vicinity
portion of the metal case 7, is indicated. Metal case 7 was
manufactured such that sealing aperture vicinity side thickness
(TC) was about 11% thicker, at 0.5 mm, than the side thickness (TB)
of the intermediate portion, with the object of improving the
sealing aperture strength.
The HV value, indicating the Vickers hardness of the sheet of 3003
alloy prior to processing of metal case 7 was 30, while the HV
value of the side wall after formation of the metal case was 58,
representing an increase in HV value due to DI processing by a
factor of 1.93.
In the process of manufacturing the DI case by this continuous
ironing process, a multiplicity of shallow grooves were formed in
the battery inside face in the direction parallel to an axis of,
metal case 7, i.e., the direction perpendicular to the bottom face.
Also, the radius of curvature R of the curved shapes constituted by
corners 8 of the battery inside face by the metal mold in the DI
case manufacturing step, i.e., the corner defined by bottom face 9
and side face 10 and the corner defined by side face 10 was 0.4 mm.
Normally, in the case of a prismatic battery, it is beneficial in
regard to the internal pressure strength for the value of this
radius of curvature R to be large; however, in order to maintain
effective internal pressure strength in a restricted effective
volume and to accommodate the elements for electromotive-force etc.
an effective manner, it is important that the radius of curvature R
should be less than 0.5 mm, and, in this embodiment, as shown in
FIG. 4, the radius of curvature R of these corners 8 was made 0.4
mm. By this means, even though the wall thickness of the metal case
is reduced, the strength in resisting internal pressure of the
battery can be maintained.
Next, a prismatic lithium ion secondary battery was manufactured
using a metal case manufactured as described above. First of all,
the positive electrode and separator and negative electrode
constituting the elements for electromotive-force were prepared.
For the positive electrode, LiCoO.sub.2 and a conductive agent
including acetylene black and a fluorinated resin binder etc. mixed
into the form of a paste, was applied on to an aluminum foil
substrate, which was then dried, pressurized and cut to form an
electrode of prescribed dimensions. A lead was mounted on this
positive electrode plate in order to make possible connection to
the positive electrode terminal of the battery. For the separator,
polyethylene micro-pore film of thickness 0.027 mm was employed.
For the negative electrode, styrene butadiene rubber (SBR) binder
and carboxymethyl cellulose (CMC) thickener etc. were added to
spherical graphite to produce a paste, which was applied on to a
copper foil substrate, which was then dried, pressurized and cut to
form an electrode of prescribed dimensions. In order to achieve
direct contact with the metal case of the battery, this negative
electrode was provided with a portion consisting solely of the
copper foil substrate of the negative electrode.
Next, the positive and negative electrodes were wound in spiral
fashion with the separator therebetween, and accommodated in the
metal case referred to above. In this case, the outermost
circumferential portion, that is wound in spiral fashion is the
portion constituted solely by the copper foil substrate of the
negative electrode, so that direct electrical contact is made
between the negative electrode and the negative electrode terminal
of the metal case. Also, connection of the positive electrode plate
and positive electrode terminal constituted by a cap of the sealed
battery was achieved by an aluminum lead. As the electrolyte, an
electrolyte was obtained by blending ethylene carbonate (EC) and
diethyl carbonate (DEC) in mol ratio 1:3 and dissolving lithium
hexafluoro phosphate (LiPF.sub.6) therein in the ratio of 1 mol/l.
This electrolyte was poured into the battery, and a sealed battery
was obtained by sealing of the aperture cap with the metal case
using ordinary laser sealing. This battery was of prismatic shape
of width 22 mm, height 48 mm, and thickness 8 mm, the battery
weight being about 18 g. The battery capacity was 600 mAh. This
battery was designated as cell C according to the present
invention.
It should be noted that this embodiment differs from the first
embodiment described above in respect of the polarity of the metal
case. Whereas in embodiment 1 described above the metal case
constituted a positive electrode and was connected to the positive
electrode plate, in this embodiment, the metal case constitutes a
negative electrode and is connected to the negative electrode
plate.
In order to compare the performance with cell C of this embodiment,
cells D and E were manufactured and evaluated as comparative
examples. The difference between cell C of the present embodiment
and cells D and E lies in a different construction of the metal
case. Specifically, the differences from cell C of the present
embodiment lie in that cell D was obtained by directly processing a
sheet of 3003 alloy of thickness 0.6 mm to a metal case having a
bottom without first nickel-plating the sheet surface, and cell E
was obtained by subjecting a sheet of 3003 alloy of thickness 0.6
mm whose surface had been subjected to nickel-plating to a
thickness of about 1 .mu.m to processing to a metal case. It should
be noted that the shapes of the metal cases of cells D and E were
the same as that of cell C of the present embodiment, and they both
shared the feature of formation of a multiplicity of shallow
grooves perpendicular to the bottom face on the battery inside face
in the DI case manufacturing step in which continuous ironing was
performed.
Conventionally, in this field of lithium ion secondary batteries,
it is well known that, in the case of a combination in which the
negative electrode is an electrode employing graphite and the metal
case contacting the negative electrode is aluminum or aluminum
alloy material, in the charging reaction of the battery, in the
condition below a certain potential, the lithium ions react with
the aluminum constituting the metal case rather than reacting with
the graphite. By this reaction, the aluminum constituting the metal
case crumbles away by formation of a chemical compound with the
lithium and the lithium is stabilized by reaction with the
aluminum, so, as can easily be imagined, making discharge difficult
and, as a result, deterioration of performance of the battery can
be predicted beforehand.
This was investigated by provoking an actual discharging reaction
using cells C, D and E. For the charging of the cells,
constant-voltage constant-current charging was performed at
20.degree. C. up to 4.2 V with a maximum of 0.5 A; discharging was
performed to a final voltage of 3 V at 20.degree. C. by
constant-current discharge of 120 mA. The cycle life of the cell
was evaluated by repeating this charging and discharging.
As a result, in the cell C of this embodiment, very stable
performance was shown as a result of a life test evaluated for up
to 500 cycles. In contrast, cell D, on the first discharge cycle,
could only be discharged to about 40% in terms of ratio to the
discharge capacity of cell C and showed further sharp deterioration
to 15% and 3% on the second discharge cycle and the third discharge
cycle, becoming completely unusable as a battery. In contrast, in
the case of cell E, while the discharge capacity ratio with respect
to cell C was about 95% on the first cycle, there was a progressive
reduction in discharge capacity on the second and third discharge
cycles to 89% and 83% and by the fifteenth cycle the discharge
capacity had become completely equal to zero. It should be noted
that in the case of both of these batteries, electrolyte leakage
and failure of the battery case occurred, by the fifth cycle in the
case of cell D and by the nineteenth cycle in the case of cell
E.
Although in the case of cell E a sheet of 3003 alloy of thickness
0.6 mm whose surface had been nickel-plated to a thickness of about
1 .mu.m was employed for the metal case, observation of the surface
of the metal case prior to constitution as a battery revealed
pinholes in the nickel in all parts, caused by the nickel plating
layer of the surface being too thin. It may be inferred that the
loss of capacity of cell E and the failure of its metal case were
the result of direct reaction of the lithium ions with the aluminum
of the metal case due to these pinholes.
The above results show that in the case of a lithium ion secondary
battery constituted with a metal case made of aluminum acting as
the negative electrode and connected to the negative electrode
plate the provision of a nickel layer on the battery inside face is
essential. Furthermore, the thickness of this nickel layer must be
such that the electrolyte and the aluminum of the metal case do not
come into direct contact including through pinholes etc.; 3.about.5
.mu.m appears necessary.
The above are embodiments of the present invention but
supplementary description is given below to further describe
aspects of the description of the above embodiments.
The bottom thickness/side thickness of the metal case whose chief
constituent is aluminum according to the present invention is
specified as 1.2.about.4.0. It might be desirable to have a higher
value in order to reduce size and weight, but if the value is made
high there are concerns regarding quality reliability and safety,
and as a of several tests it was found that a range up to 4.0 is
satisfactory. Also if this value is less than 1.2, the benefit in
terms of raising battery energy density is insufficient. Although
in the embodiments 3003 alloy was selected from Al--Mn based alloy
(3000 type) constituting wrought product of non-heat treated alloy
as the material whose chief constituent is aluminum that is
employed herein, in the present invention various aluminum
materials known as pure aluminum (JIS1000 grade) or aluminum alloy
(JIS3000 or 4000 grade etc.) may be employed.
Next, it is a characteristic of the present invention that a
multiplicity of shallow grooves perpendicular to the bottom face
are formed in the battery inside face of the battery metal case and
the depth of these grooves is preferably 0.5.about.10 .mu.m.
Also, it is beneficial to provide a nickel layer of under 30 .mu.m
on the battery inside face of the metal case whose chief
constituent is aluminum. This is because, whereas a construction in
which the aluminum of the metal case is directly in contact with
the electrolyte within the battery represents an impracticable
battery system from the point of view of corrosion resistance, by
providing a nickel layer of at least 3.about.5 .mu.m but less than
30 .mu.m on the battery inside face in such a battery system, the
problem of corrosion resistance can be solved and the benefit of
enabling the use of lightweight aluminum can be achieved. It is
also beneficial to provide a nickel layer of up to 30 .mu.m on the
battery outside of the metal case whose chief constituent is
aluminum. The strength of the lead connections when a plurality of
batteries are connected as a pack can thereby be raised.
Furthermore, if, in regard to the thickness of the side wall of the
metal case, the side thickness (TC) in the vicinity of the battery
sealing aperture is made at least 10.about.30% thicker than the
side thickness (TB) in the other portions, the benefits of the
present invention can further emphasized. This is because ability
of the battery to withstand internal pressure can be comparatively
satisfactorily maintained even if the side thickness of the metal
case is made quite thin. Rather the location where problems occur
in such batteries in regard to pressure withstanding strength is
the vicinity of the battery sealing aperture. In order to improve
the pressure withstanding strength of the vicinity of the sealing
aperture of batteries in which such pressure withstanding strength
is a problem, it is effective to make the side thickness in the
vicinity of the battery sealing aperture (TC) thicker than the side
thickness in other portions (TB). By making it at least
10.about.30% thicker, it is possible to improve the balance of the
whole by seeking thickness reduction of the metal case as a whole
while yet ensuring necessary thickness of the vicinity of the
battery sealing aperture which is important for pressure
withstanding strength.
Moreover, with future increases in battery energy density, battery
sizes continue to move in the direction of progressive
miniaturization and reduction in overall thickness. In these
circumstances it is desirable to make the thickness of the side
wall of the metal case as small as possible. With the DI process of
the present invention, a technical response to such requirements is
feasible and the result has been achieved that small side thickness
which was conventionally considered to be the limit with the impact
process and transfer drawing process is being obtained. By this
means, the thickness of the side wall of the metal case can be
reduced to a level that was hitherto unavailable, enabling
batteries of even higher energy density to be realized.
Although in the embodiments described above the examples of a
cylindrical and prismatic lithium ion secondary battery were
employed, the present invention can be applied apart from these to,
for example primary batteries such as alkali manganese dry
batteries or lithium primary batteries, or polymer lithium
batteries and also to alkali accumulators exemplified by nickel
cadmium accumulators or nickel-hydrogen rechargeable batteries
etc.; in fact it can be applied to primary batteries or secondary
batteries wherein the metal case is of cylindrical shape, prismatic
shape or shape similar thereto, so long as they are batteries in
which the elements for electromotive-force are accommodated in a
metal case.
As described above, by means of the present invention, a high value
that was previously unavailable of the bottom thickness/side
thickness ratio of a metal case, whose chief constituent is
aluminum, can be achieved. By this means, a battery combining both
high battery energy density and high reliability together with
safety, which was problematical with conventional batteries, can be
provided; this is therefore useful as a technical response to
requirements for battery size miniaturization and thickness
reduction.
* * * * *